Ok, here are my €0.02. Note the Euro sign, so it might be just one cent tomorrow. I'm sorry in advance if it's n00b advice to you; just trying to help out.
- Check EQ settings. Turn any / all EQ-ing off
- Check mute and /or ATT settings (if any). Some receivers allow you to set mute to a manual level, such as -40 DB. Then again, this should show up on your display!
- Check / change the interconnects
- Try a different discrete input of the receiver / Try a different discrete output of your source device.
- Check if your amp isn't set to any bi-amp setting (called speaker-c with Marantz equipment) while you're not actually bi-amping / bi-wiring. Check if you're bi-sexual while you're at it.
- Try a different speaker pair (any other pair, really)
you didn't specify your source material. So here goes my best tip (that's €0.01 out of the €0.02 I'm throwing at ya)
- If you've connected a computer, iPod, or any other equipment with its own volume control: Turn the volume of these machines all the way UP. Then set the receiver volume accordingly. Please note: You can set the volume level of your sound card software as well as iTunes volume level. This means you've got a possible 3 volume settings, including the one on your receiver.
- Set sound card software to "headphone" or stereo mode. Many multi channel sound cards have DSP software settings to playback stereo as 5.1 music.
- Disable any DSP settings on your amp. Or playback the source material using a "direct source" setting of your amp.
- Many sources allow their own set of EQ / volume control. Make sure they are disabled. This is true for DVD players as well as iTunes, Windows Media Player, and other software / hardware.
- Check volume levels when listening to the internal FM/AM radio. Does it sound different from the other sources you're listening too?